It sounds like this“The Moon is too far away for Earth's gravity to reach it, so it just floats there.”
Thinking gravity stops at some distance
Why it feels right
Gravity feels like something that belongs to the ground. You feel it where you stand, and you see astronauts floating about as if it had been switched off up there. Pictures of the Moon show a lot of black between it and us. It is natural to think there must be a height where Earth's pull gives out, and the Moon sits beyond it. Nothing in everyday life ever shows you gravity getting weaker, so the idea of an edge is easier to picture than a pull that thins out and never ends.
Why it’s wrong
Gravity never switches off. It weakens with distance, and at the Moon it is about 1 part in 3,600 of its strength at the surface, but that small pull is the only thing bending the Moon's path, so it is all that is needed.
A worked example
Take the pull you feel at the surface, 9.8 metres per second every second, as 1. Go to twice as far from Earth's centre and the pull is a quarter of that. Ten times as far, a hundredth. The Moon is about 60 times as far from Earth's centre as you are on the ground, so its pull is 1 part in 60 x 60, which is 1 part in 3,600: about 0.0027 metres per second every second.
That is tiny, but look at what it has to do. Nothing else is acting on the Moon. Every second the pull bends its path a little, by about 1.4 millimetres, and over a month those small bends add up to a full circle. A pull of 1 part in 3,600 is plenty when it is the only pull there is and nothing ever resists it.
The space station is only 400 km up, about 6% further out than the ground, and Earth's gravity there is still about 90% of what you feel at home. The astronauts float because they and the station are falling together, not because gravity has gone.
How to catch yourself
- You say something is 'beyond gravity' or 'out of gravity's reach'.
- You explain floating in space as having no gravity.
- You assume a weak force must be too weak to matter, whatever else is (or isn't) acting.
What to hold instead
Gravity has no edge. It weakens with the square of the distance, so it gets very small but never reaches zero. At the Moon it is about 1 part in 3,600 of the pull at the surface, and with nothing else acting on the Moon, that is exactly enough to keep bending its path round Earth.
Where you’ll meet it
- Explaining why astronauts float on the space station.
- Understanding why the Moon, and the satellites behind phone signals and weather forecasts, stay in orbit.
- Seeing how the Sun's gravity holds planets billions of kilometres away.
Questions people ask
- Is there gravity in space?
- Yes. Gravity is still about 90% as strong at the height of the International Space Station as on the ground. Astronauts float because they are in free fall around Earth, not because the pull is missing.
- How far does Earth's gravity reach?
- In principle without limit: it weakens with the square of the distance but never becomes zero. Earth's pull still acts on the Moon, and to a tiny degree on spacecraft millions of kilometres away, where other bodies' gravity simply becomes larger.